Dietary calcium (Ca) is likely an important determinant of peak bone mass in young adults. Our previous studies suggest it influences skeletal formation during adolescence. A high peak bone mass at skeletal maturity may prevent subsequent postmenopausal and senile osteoporosis. We have also provided a metabolic explanation for our hypothesis by demonstrating that teenage females on a high Ca diet are able to retain Ca rather than excrete it renally. The results of our research highlight the need for a definitive intervention study in the form of a randomized, double-blinded, placebo-controlled trial to evaluate the effect of Ca on bone mass during the period of rapid bone modeling. This is particularly important in view of the fact that a substantial proportion of adolescent females consume far below the RDA for Ca. Therefore, this project will assess the effects of increased Ca intake on bone mass in females during puberty. The hypothesis is that bone mass may be optimized by increasing Ca intake, that Ca may be an optimal early prophylactic therapy for teenage females against subsequent osteoporosis. Three hundred Caucasian females in Tanner's SMR (sexual maturity ratings) stage 2 will be randomized into groups of 150. Group A will receive placebo, and Group B will receive 1 g of supplemental Ca from Ca citrate-malate. Calcium efficacy will be assessed throughout the 4-year study. Calcium efficacy will be assessed by food record, anthropometry, blood and urine chemistry, and bone mass measurements every 6 to 12 months. In addition, hand radiographs taken at the beginning and at the end of the study will serve to assess skeletal age as well as potential genetic influence. A detailed assessment of true Ca bioavaolability in a subset of each group (N= 12) will be performed annually by a classical balance technique. Calcium absorption will be investigated concurrently by the use of stable Ca isotopes. Bone turnover indices will be obtained throughout the study to provide information regarding the relationship between the accumulation of bone mass and the rate of bone modeling. This four-year, longitudinal trial (in conjunction with our previous epidemiologic study) will provide unique information about the prophylactic role of Ca in osteoporosis by maximizing bone mass during the critical years of adolescence, when Ca requirements are highest and Ca intake declines. The models of transferability of bone mass from both parents will be elucidated as well.